Cosmology used to be called the hunt for two numbers. It was simple and elegant. Nowadays we need at least six. It is neither simple nor elegant. So how did we get here? The two Big Numbers are, or at least up till the early-90s were, the Hubble constant H ₀ and the density parameter Ω. These told us Everything. Or so we thought. The Hubble constant is the expansion rate of the universe.